Advice on oil leak(s)

[cliffmac]

Truth here...today I went to my local AutoZone to get a quart of Castrol 20-50 for my 1990 300TE. This thing leaks (does not burn) about 1 quart every 6 weeks or so. Anyway, when I came out of the store I noticed (which I didn't notice on the way in) there were 2 other hoods up in the parking lot under which the owner was adding oil. One was an S320 and the other a C280...I go and do the same to my 300TE and I wonder...do all these things leak oil? It was semi Twilight Zone-ish. Not only that, but we all looked at each other, made eye contact, and left. How much would
a front crankshaft seal replacement go for? Thanks